The field of the invention is nuclear magnetic resonance (NMR) imaging methods and systems. More particularly, the invention relates to a method and apparatus for formation of a gradient coil with reduced power deposition. It will be appreciated, however, that the invention is also amenable to other like applications.
When a substance such as human tissue is subjected to a uniform magnetic field (e.g., a polarizing field B0in the z direction), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (e.g., an excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, G y, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The use of gradient coils to generate a gradient field about the bore of a magnet for imaging is known in the art of nuclear magnetic resonance imaging. Generally, a patient is positioned on an examination table and inserted into a bore of a magnet. The magnet provides a uniform magnetic field B0 throughout the bore. The gradient coils extend around the bore and are energized to impose time varying magnetic fields on the uniform magnetic field.
Conventional gradient coils have a fixed field-of-view (FOV). It is generally well known that the larger the FOV, the lower the efficiency rating for a respective coil. That is, a gradient coil with a large FOV requires a higher voltage times ampere product from the gradient amplifier to produce a given gradient strength times gradient slew rate product than a gradient coil with a small FOV. Additionally, since the peak magnetic field, for a given gradient strength, is larger with large FOV coils, for a given slew rate, high dB/dt is associated with larger FOVs, which can result in increased peripheral nerve stimulation (PNST). Hence imaging protocols requiring high gradient power and high slew rate are generally performed on MRI systems equipped with a small FOV gradient set.
Due to the above-mentioned physiological effects on the patient, constraints are placed on maximum switching speeds (slew rate) for the gradient fields. Time-varying magnetic fields induce currents in conductive materials and rapidly changing field gradients can induce currents in a patient being imaged. Under some circumstances, these induced currents can stimulate nerves, a phenomenon known as peripheral nerve stimulation (PNST). Therefore, every MRI employed for human patients must conform to one or more magnetic field rate of change limitations in accordance with regulations from the FDA and other regulatory agencies. Thus, current MRI systems, therefore, limit the gradient slew rates accordingly.
All gradient coil designs intended for human use will have physiologic limits. The slew rate, which gives the limit, however, will depend on the effective length of the coil. Note that, the effective length L is not necessarily the true length of the coil. The effective coil length is the ratio of the maximum field strength (mT) found within the gradient coil divided by the applied gradient strength (mT/m). While the effective coil length has units of length, it does not relate to any physical dimension within the coil. It should not be confused with the distance from the iso-center of the gradient coil to the location of maximum field variation. Maximum field strength is defined as the vector sum of all three components of field produced by the gradient coil axis.
A number of improvements have been developed to provide more than one FOV for the gradient field in MRI systems. One approach is to integrate two sets of gradient coils on one system to provide two distinct FOV sizes. Manufacturing a coil with this approach is relatively straightforward, however, coil efficiency is greatly reduced. So-called twin coil designs require multiple coils and their respective shields to be stacked within a limited space. However, coil efficiency is dependent upon separation of the shield coil from the primary coil. Therefore twin coil designs result in less efficient operation. The UK patent application GB 2,295,020 describes a modular gradient coil system that unites, in one coil body, a gradient coil for rapid measuring sequences and an activatable gradient coil for conventional measuring sequences. The gradient coil for rapid measuring sequences has a small linearity volume and allows rapid switching of gradient fields. In the joint operation of the two coils, the gradient coil system has a large linearity volume for conventional measuring sequences with slowly switched gradient fields.
Another approach requires the disabling or enabling of certain parts of the gradient coil windings to adjust the FOV. U.S. Pat. No. 5,311,135 teaches a gradient coil for a magnetic resonance device which has four saddle-shaped coils, each of which has first and second terminal points respectively at the beginning and end of its conductor, as well as at least one tapping point between the terminal points. The arrangement also includes a switching mechanism, so that each of the two coils can be supplied with current either between the terminal points or between the first terminal point and the tapping point. In this way, at least two different linearity volumes of the gradient coils can be set, for instance corresponding to a size of a region that is being imaged. Advantageously, here, multiple fields of view are obtained from primary and shield windings each on only one surface, with maximal separation. However, the respective windings are each divided into different circuits, capable of being individually supplied with current. When these circuits are driven such that their generated fields reinforce each other, the large field of view mode is achieved. The small field of view (high slew rate) mode is achieved when only one circuit is used, or the fields are opposed. Advantageously, this type of coil is more efficient than a xe2x80x9ctwinxe2x80x9d coil. However difficulties include the reduced freedom in terms of optimizing the field linearity; and also the difficulty in coil construction, due to the need for multiple connections between windings.
Similarly, a gradient coil with at least two independently controllable portions, with multiple control states for generating a gradient field for imaging multiple regions is described in article: Gradient System Providing Continuously Variable Field Characteristics, Magnetic Resonance in Medicine, 47:800(2002) by Kimmlingen et al. In this patent, by controlling gradient fields for at least two imaging sub-regions, with neither of the two regions being a subset of the other, it is possible to pick up MR images for a larger aggregate imaging area, which derives at least from the sum of the two imaging sub-regions, using rapid, high-resolution measuring sequences without triggering stimulations.
Configurations of gradient coils that employ extra xe2x80x9ctwinxe2x80x9d or excess coils are costly and complicated. Moreover, xe2x80x9ctwinxe2x80x9d or dual field of view gradient coils are less efficient than single field of view systems. Generally, only one coil can be designed to be efficient, but typically, both are compromised. Additionally, gradient coil configurations that employ separate circuits are generally efficient for one field of view, and yet less efficient for another field of view. Inefficient gradient coil configurations require additional space and result in excess heat generation. Therefore, there is a need in the art for a gradient coil design that provides improved efficiency and satisfies the need for multiple fields of view, and current regulatory requirements for slew rate, without excessive complication and cost.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by the embodiments as disclosed herein.
A method of configuring a gradient coil in a variable field-of-view gradient coil assembly for use with an MR imaging system comprising: determining a first current density corresponding to a first field-of-view for an effective gradient coil; computing a second current density corresponding to a second field of view, by reducing current from the first current density without changing the sign of said current associated therewith; determining a geometry for a main gradient coil to provide said second field of view; and ascertaining a geometry for a corrector coil.
Also disclosed herein is method reducing power deposition in a variable field-of-view gradient coil assembly for use with an MR imaging system comprising: determining an effective gradient coil configuration corresponding to a first field-of-view; determining a geometry for a main gradient coil for a second field of view; ascertaining a geometry for a corrector coil; and ensuring that a return conductor for the main gradient coil and the corrector coil substantially overlap.
Another disclosure herein is a gradient coil assembly with variable FOV for use with an MR imaging system, the coil assembly comprising: a main gradient coil disposed about an imaging axis to produce a gradient field; a corrector coil disposed about an imaging axis and positioned with a return portion substantially overlapping a return portion of main gradient coil. The main gradient coil and said corrector coil operate in conjunction to provide a first field of view and the main gradient coil operates to provide a second field of view.
Yet another disclosure herein is for an MRI system comprising: a magnetic resonance imaging (MRI) system with a gradient coil assembly for variable field of view imaging. The gradient coil assembly comprising: a main gradient coil disposed about an imaging axis to produce a gradient field; a corrector coil disposed about an imaging axis and positioned with a return portion substantially overlapping a return portion of the main gradient coil. The main gradient coil and the corrector coil operate in conjunction to provide a first field of view and the main gradient coil operates to provide a second field of view.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.